1. Field of the Invention
The present invention is directed, generally, to a battery assembly, and more particularly, to a thin-film rechargeable battery and its method of manufacture.
2. Description of the Invention Background
A typical thin-film rechargeable battery incorporates a cathode current collector, a cathode, an anode current collector, an electrolyte, and an anode, in series, as layered components deposited over a substrate. The arrangement of the layered components and the materials that comprise each individual layer play an important role in determining the specific capacity, the utility, and performance of the battery cell.
The substrate may be selected from various materials, but is typically a glass, polymer, or ceramic. The choice of the substrate depends upon, among other factors, the processing conditions during manufacturing, such as, for example, temperature and reactive environment. For example, where the battery requires a low-power application suitable for amorphous cathodes, the substrate may be selected from a wide range of inexpensive materials including glass and polymers. On the other hand, ceramic substrates are typically used for high-power applications because high temperature annealing under oxidizing environments is required in order to obtain crystalline cathodes.
FIGS. 1 and 2A-2F illustrate a typical prior art thin-film rechargeable battery 10 and its method of manufacture. The layered components that form the battery 10 are deposited over a ceramic substrate 2 and include, in series, a cathode current collector 4, cathode layer 6, an anode current collector 8, an electrolyte layer 12, an anode layer 14, and a protective coating 16. For high-power applications, the manufacturing process includes an annealing step that follows the deposition of the cathode layer 6. Annealing the deposited cathode layer is necessary because, at ambient temperature, the cathode layer is typically amorphous and lacks the crystallinity and the conductance necessary for high-current requirements. It is the high temperature and other extreme processing conditions associated with the annealing step that typically limits the choice of substrate materials to ceramic, rather than glass or polymers, for high-power battery applications.
Although prior art battery designs are adequate for use with thin-film rechargeable batteries, several deficiencies exist in the prior art that limit their effectiveness. For example, formation of the layered components over the substrate is relatively time consuming as each of the six formed layers requires a separate manufacturing step. In addition, the arrangement of the layered components, in some cases, provides ineffective insulation between the active layers and inefficient battery utility and performance due to reactivity between the individual layers. Furthermore, for high-power applications, the fabrication method is, most often, limited to a "wafer-by-wafer" process because of the brittleness of the ceramic substrate. Because the thin ceramic materials are very fragile, there exists a lower limit in substrate thickness that must be employed to ensure sufficient durability. As a result, an intrinsic area-to-volume ratio limitation is created, which, in turn, results in specific capacity and energy limitations. Ceramic substrates are, also, relatively expensive and, therefore, comprise a significant portion of the overall manufacturing cost.
Accordingly, the need exists for an improved thin-film rechargeable battery and method of manufacture that, for example, may provide one or more of greater area-to-volume ratio for increased specific capacity and energy output, greater substrate strength for greater manufacturability, and increased manufacturing efficiency with diminished manufacturing cost.